See the video here: http://www.reddit.com/r/trueminecraft/comments/14gqd7/myth_busting_how_do_zombie_pigmen_become/
Zombie pigmen are supposed to calm down after a while. As the video shows, the only way to clear angry pigmen is to wait for them to despawn.
Comments 3
I have encountered this in version 1.5.2. I have not yet entered The Nether in version 1.6+
If a hostile zombie pigman picks-up a dropped item from a player (death) such as armour, they remain hostile (at least 15 minutes later - still angered) and will not despawn (intended behaviour).
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Is this still a concern in the current Minecraft version? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.